Welcome, Guest. Please login or register.
Did you miss your activation email?

News: The latest PSX update (version 10.25 from 14 February 2018) is available at: http://aerowinx.com/board/index.php?topic=4191.0

Author Topic: CDU standby navigation  (Read 527 times)

Hardy Heinlin

  • Moderator
  • Join date: May 2009
  • Posts: 8758
    • Aerowinx
    • Email
Re: CDU standby navigation
« Reply #20 on: Mon, 12 Feb 2018 17:47 »
In case of a single FMC failure, with FMC L operative, I'm seeing two possibilities:

1. All CDU route memories get data from the operative FMC, independent of the NAV source selections.

Or:

2. CDU C memory gets data from the operative FMC; CDU L memory gets it if captain's NAV source is set to FMC L or CDU L; CDU R memory gets it if F/O's NAV source is set to FMC L or CDU R. (If the source selection is relevant, at least the selection of CDU C or of the inoperative FMC should disconnect the link.)

Just got a good hint from a trustworthy expert. I now take possibility #2 -- with a slight variation:

CDU C memory gets data from captain's NAV source selected FMC
CDU L memory gets data from captain's NAV source selected FMC
CDU R memory gets data from F/O's NAV source selected FMC


mark744

  • Join date: Feb 2015
  • Posts: 31
Re: CDU standby navigation
« Reply #21 on: Mon, 12 Feb 2018 18:35 »
Sounds like a good plan

What if either pilot has the unserviceable FMC selected?    Map fail on ND? and Timeout/Menu on CDU?
until a valid input is selected? ie the good FMC or CDU s?

What affect should the FMC Master Switch have in this situation?
The QRH for a single FMC fail calls for the Master FMC Switch to be set to the operable FMC, so it must have an affect. What if it is set incorrectly?
(Is it selection of FMC used for FCC and  EECs ?)
« Last edit: Mon, 12 Feb 2018 18:53 by mark744 »

Hardy Heinlin

  • Moderator
  • Join date: May 2009
  • Posts: 8758
    • Aerowinx
    • Email
Re: CDU standby navigation
« Reply #22 on: Mon, 12 Feb 2018 21:28 »
All these what-if questions are answered by PSX. Just try it in PSX and you will see :-)

I will only change that nuance discussed in the original post. The rest is correct, I'm sure.

Hardy Heinlin

  • Moderator
  • Join date: May 2009
  • Posts: 8758
    • Aerowinx
    • Email
Re: CDU standby navigation
« Reply #23 on: Tue, 13 Feb 2018 01:34 »
CDU C memory gets data from captain's NAV source selected FMC
CDU L memory gets data from captain's NAV source selected FMC
CDU R memory gets data from F/O's NAV source selected FMC

These modifications are now available in PSX 10.24:

http://aerowinx.com/board/index.php?topic=4191.0


|-|ardy

mark744

  • Join date: Feb 2015
  • Posts: 31
Re: CDU standby navigation
« Reply #24 on: Tue, 13 Feb 2018 12:38 »
Thanks Hardy for all the mods on this

A few comments from an old Boeing manual:

Boeing Operating manual
 quote A
"The CDUs do not have a performance or navigation database. The CDUs continuously copy the active route from the FMC."

......At the moment in PSX, with both FMCs working and just F/OsNav Source Sel to CDU R, CDU R is not  'continuously" updating from any FMC


quote B
"If both FMCs fail, the CDUs retain flight plan waypoints except for conditional waypoints"

......so  they only retain the waypoints they have, at the point when BOTH fail (until then, they continue to copy from the FMC)

quote C
"Failure of the primary FMC causes both CDUs to display the MENU page and the scratchpad message TIMEOUT-RESELECT"

......I assume this is because the L FMC is the master and both CDUs are trying to display info from that FMS when it fails
If the Master FMC switch were then to be moved to R FMC, then I assume you get the behavior in quote D below

...this is not reflected in PSX at the moment



quote D

"Failure of the secondary FMC displays the respective CDU MENU page and the scratchpad message TIME-OUT RESELECT. Rotating the respective Navigation Source Selector to the operable FMC restores the CDU display and NDs to normal"

.......this is happening just like this in PSX


Thoughts anyone?


« Last edit: Tue, 13 Feb 2018 13:44 by mark744 »

Hardy Heinlin

  • Moderator
  • Join date: May 2009
  • Posts: 8758
    • Aerowinx
    • Email
Re: CDU standby navigation
« Reply #25 on: Tue, 13 Feb 2018 15:50 »
Quote
Boeing Operating manual
 quote A
"The CDUs do not have a performance or navigation database. The CDUs continuously copy the active route from the FMC."

......At the moment in PSX, with both FMCs working and just F/OsNav Source Sel to CDU R, CDU R is not  'continuously" updating from any FMC

In PSX the CDUs do not have a performance or navigation database either. So PSX is correct here.
Yes, in PSX the CDUs continuously copy the active route from the FMC when the related NAV source selectors link the related CDU with an operative FMC. This info in bold font is new, and modelled in PSX 10.24 now. This info is the most trustworthy info I have (less vague than other infos); it's being teached in an airline based on a large european island, if you see who I mean :-)

The Boeing Operating manual just says "continuously copy", but it doesn't tell you when. It's just a general, simplified statement like "eagles fly continuously", which is correct. But when do they fly continuously?


Quote
quote B
"If both FMCs fail, the CDUs retain flight plan waypoints except for conditional waypoints"

......so  they only retain the waypoints they have, at the point when BOTH fail (until then, they continue to copy from the FMC)

Yes, they copy from the operative FMC when the NAV source selectors are set as instructed by the QRH (see bold font above).


Quote
quote C
"Failure of the primary FMC causes both CDUs to display the MENU page and the scratchpad message TIMEOUT-RESELECT"

In PSX, the CDU which is linked with the remaining operative FMC will not return to the MENU page as this CDU is already linked with the operative FMC (e.g. CDU R with FMC R), regardless of the FMC master switch position. This behaviour in PSX is based on observations on a real 744 parked on the ground. I recall it was differently modelled in an earlier alpha version based on manuals, and then remodified after this real-deck observation. The FMC master switch position is also irrelevant to the EICAS thrust limit indications. It's relevant to the autothrottle (and LNAV/VNAV get a yellow line). I can remodify this detail again. But not before there is 100% evidence that this is a general, non-airline-specific feature.


Cheers,

|-|ardy

mark744

  • Join date: Feb 2015
  • Posts: 31
Re: CDU standby navigation
« Reply #26 on: Tue, 13 Feb 2018 16:50 »
Hardy,

Thanks for taking the time to explain.

Maybe my manual was never updated, airline specific, or just plain wrong.
I'm sure your inside info has PSX absolutely correct now.

Regards,
Mark.

Hardy Heinlin

  • Moderator
  • Join date: May 2009
  • Posts: 8758
    • Aerowinx
    • Email
Re: CDU standby navigation
« Reply #27 on: Tue, 13 Feb 2018 17:09 »
Don't give up :-) I'm not saying your manual is wrong. I'm just saying that those words "continuously copied" don't tell the "when". Those words per se are not wrong, they're just incomplete, they just tell one part of the truth.

Regarding that return to the MENU page: I can't gurantee that this is the final truth for all aircraft for all times. It's just what I have learned so far. It may disagree with other observations. My philosophy is the method of falsification; we can never know the final truth of all things at all times, so we can never claim the absolute truth. We can only keep the latest theory until someone discovers something different. Every step is just another theory (a better one, but never the final one).


Regards,

|-|ardy
« Last edit: Tue, 13 Feb 2018 17:20 by Hardy Heinlin »

Jeroen Hoppenbrouwers

  • Moderator
  • Join date: May 2009
  • Location: KTMB
  • Posts: 3004
  • Hoppie designs SATCOM equipment for airliners.
    • http://www.hoppie.nl/
    • Email
Re: CDU standby navigation
« Reply #28 on: Tue, 13 Feb 2018 19:55 »
Background: you get TIMEOUT-RESELECT three seconds after the MCDU display feed of the currently selected subsystem stops sending keepalive data (words 172 and 377 basically). It happens also for ACARS, SATCOM, etc. Possibly this gives another clue on what exactly is connected through which switches/relays.


Hoppie

Phil Bunch

  • Join date: May 2009
  • Posts: 1128
Re: CDU standby navigation
« Reply #29 on: Tue, 13 Feb 2018 22:16 »
From Hardy:

Regarding that return to the MENU page: I can't gurantee that this is the final truth for all aircraft for all times. It's just what I have learned so far. It may disagree with other observations. My philosophy is the method of falsification; we can never know the final truth of all things at all times, so we can never claim the absolute truth. We can only keep the latest theory until someone discovers something different. Every step is just another theory (a better one, but never the final one).

-----------
I've come to believe that physics also works this way.

Karl Popper was one of the best known advocates of this philosophy.  Science develops testable theories, and progress occurs when experimental evidence shows that the theory is wrong.  A theory is not shown to be correct, but can only be falsified. 

From wikipedia, an excerpt:

https://en.wikipedia.org/wiki/Karl_Popper

Popper is known for his rejection of the classical inductivist views on the scientific method, in favour of empirical falsification: A theory in the empirical sciences can never be proven, but it can be falsified, meaning that it can and should be scrutinized by decisive experiments. Popper is also known for his opposition to the classical justificationist account of knowledge, which he replaced with critical rationalism, namely "the first non-justificational philosophy of criticism in the history of philosophy."
-------------------

It's interesting that PSX is being improved in much the same way as modern physics. 

These comments are meant to be complimentary to PSX and its continuing development.

(FYI - I'm a <retired> physical scientist.)
Best wishes,

Phil Bunch

John H Watson

  • Join date: May 2010
  • Location: On a pedestal
  • Posts: 1546
    • Email
Re: CDU standby navigation
« Reply #30 on: Tue, 13 Feb 2018 22:39 »
Quote
Maybe my manual was never updated, airline specific, or just plain wrong.

Quote
Regarding that return to the MENU page: I can't gurantee that this is the final truth for all aircraft for all times. It's just what I have learned so far. It may disagree with other observations.

Not sure if we are talking about the same thing, but my FMC fail ground observations seemed to vary from aircraft to aircraft (within the same fleet). At one point, I had a theory that the results varied with IRU alignment (but I can't explain why it would affect the results). Also, aircraft system behaviour does differ on the ground to behaviour in the air for certain systems, so anything is possible.

At one point we were pulling CBs for the FMC prior to every flight to flush out the data in the ATC (?) logs. Too much data was locking up the FMCs. However, this usually involved pulling both FMC CBs at the same time. 

Jeroen Hoppenbrouwers

  • Moderator
  • Join date: May 2009
  • Location: KTMB
  • Posts: 3004
  • Hoppie designs SATCOM equipment for airliners.
    • http://www.hoppie.nl/
    • Email
Re: CDU standby navigation
« Reply #31 on: Wed, 14 Feb 2018 00:53 »
(FYI - I'm a <retired> physical scientist.)

We are all engineers -- retired or not   :-)


Hoppie

When people speak about "mad scientists", they usually really mean "mad engineers". Real scientists do not build working machines of greatness -- they publish what-if games. Many so-called scientists are actually engineers, mad or not.






:-D

torrence

  • Join date: May 2009
  • Location: Altadena
  • Posts: 355
Re: CDU standby navigation
« Reply #32 on: Wed, 14 Feb 2018 02:48 »
In my world (space/planetary science) we get a real chuckle out of the label 'rocket scientist'.  There is nothing such as a rocket scientist - just brilliant rocket engineers who make it possible for us scientists to get some interesting data to mess around with  :).  I bow down before their altar. 

Cheers,
Torrence
Cheers
Torrence

Will

  • Join date: May 2009
  • Location: Chicago
  • Posts: 1574
    • Email
Re: CDU standby navigation
« Reply #33 on: Wed, 14 Feb 2018 16:24 »
"This task should be easy for you. It's not rocket surgery after all."
Will /Chicago /USA